Dispersion of inorganic fine particles has been used as printing ink or functional paste which is used in the steps for the manufacture of, for example, capacitor, inductor, electronic circuit, electrode, secondary battery, fuel cell, ceramics and display, in the field of electronic materials, or as a part of starting materials to be blended, in the field of general industrial paint or ink.
When used by the printing method or the application method in the steps for the manufacture of electronic materials, a dispersion of inorganic fine particles makes it possible to precisely control the machining of electronic materials. Hence, dispersion of inorganic fine particles has been industrially utilized as a method to contribute to the high performance and mass production of electronic materials.
In the manufacture of materials by precision machining such as electronic materials, dispersion of fine particles has usually been printed or applied with a printer, a coater or a dispenser. For example, a circuit pattern can easily be formed on a base if circuit is printed with a screen printing method by use of a conductive coating liquid which contains metal fine particles dispersed therein. Furthermore, a protective layer or an insulating layer having a large area can be formed easily by the application, with a coater, of a coating liquid which contains metal oxide particles dispersed therein. When a dispenser is used for the application of a coating liquid which contains metal oxide particles dispersed therein, it becomes possible to apply a small amount accurately.
When a dispersion of fine particles is applied by the printing method or the coating method, the fluidity of the dispersion of fine particles has sometimes caused the deformation or distortion at the edge of print or coating. The fluidity of a dispersion of fine particles has also caused a problem as insufficient thickness of printed film or of coating film.
Due to recent development of manufacturing technology, it has been required to produce the finer or smaller materials in a more efficient manner. As a result, it has been required to have thicker printed film and to increase the precision of the shape at the edge of printed film or coating film, both of which have a large influence on performance.
The aforementioned problems can be solved by enhancing the thixotropy of dispersion of fine particles. The present technology, however, is still insufficient to provide a complete solution because of the occurrence of other problems which are mentioned below. The term thixotropy which is used herein means the property that, when viscosity is measured with a rotational viscometer for example, low viscosity is indicated at a high rotation (high strain rate) and high viscosity is indicated at a low rotation (low strain rate).
Patent Document 1 uses, as a thixotropic agent by which to keep the shape of circuit pattern, fatty amide wax, fumed silica and organic bentonite. When added in a large amount, however, these thixotropic agents may cause clogging in a mesh screen since they are solid particles. Furthermore, fumed silica which is an inorganic substance and organic bentonite are hard to be removed in calcination process.
The thixotropic agent not containing solid particles which is disclosed in Patent Document 2 has a high solvent content. Thus produced dispersion of fine particles has therefore sometimes caused inconveniences due to the solvent of thixotropic agent, such as the mixing of other solvents or the decrease of content of fine particles.
On the other hand, zinc rich paint which is an industrial paint is a high concentration dispersion of zinc powders. In order to prevent the precipitation of zinc powders, zinc rich paint contains a conventional viscosity modifier such as fatty amide wax, fumed silica and organic bentonite. However, said conventional rheology modifier often causes excess increase in viscosity when imparting high thixotropy which is necessary in order to prevent precipitation sufficiently.
Coating film made from inorganic zinc rich paint has usually a large number of voids. On this account, when a base coat film or a top coat film is formed on the same, air in the voids causes defects in coating film such as bubbling and pin hole. To prevent these defects, there has heretofore been made, before said base coat film is applied, a so-called “mist coat step” which comprises applying a low viscosity liquid which has been prepared by diluting the base coat paint with a large amount of solvent, with a view to expelling air from the voids of the coating film of inorganic zinc rich paint. The above-mentioned mist coat does not, however, overcome the above-mentioned defects completely. Also from the viewpoint of workload reduction, it has been required to inhibit the occurrence of bubbling without a mist coat step (Patent Document 3).
Patent Document 4 discloses an inorganic zinc paint composition which comprises a silicate vehicle, zinc powder and a flake pigment. When a coating film is formed by using this paint composition, the flake pigment is distributed selectively in the surface layer of the coating film. As a result, solvent cannot reach the voids and replace air in the voids, and, thus, the formation of bubble or pinhole in the coating film can be prevented.
In the case of application on a vertical face, however, bubbles may be formed in the top coating film since it is difficult in the above-mentioned method for the flake pigment to be distributed selectively in the surface layer of the coating film. In the case of spray application, there has been a problem that alles tip, filter or strainer in the spray coater is clogged with flake pigment.